Push to talk over Cellular (PoC) is a new telecommunication service that enables real-time one-to-one and one-to-many (group) voice communication in a cellular network. PoC can be provided as a packet-based user or application level service in a digital communication system. In PoC, the underlying communication system provides the basic connections (i.e. IP connections) between the communications applications in user terminals and a communication service.
Due to the great interest in the PoC services, individual vendors have provided early adoptions of the emerging technology, primarily in the form of standalone PoC systems. Quite recently, a group of interested organizations prepared an industry specification for PoC, with the aims of following existing 3rd Generation Partnership Project (3GPP) IP Multimedia Subsystem (IMS) specifications. The standardization work to this direction has since then continued in Open Mobile Alliance (OMA) using the existing set of specifications as a starting point. A shared interest is presently to integrate the separate existing and future PoC systems in such a way that PoC users could utilize the service in wide areas and among a large subscriber base without continually concerning themselves with the separately operated administrative domains.
A PoC communication service is typically implemented with a communication server system while client applications reside in the user equipment or terminals. Establishment of connections in a PoC system is implemented using the mechanisms of a Session Initiation Protocol (SIP). The SIP protocols comprise querying routing information for the signalling messages from defined databases of the system. During establishment of sessions, such queries are generally implemented based on the identity of the calling and/or the called subscriber and therefore any queried server, in this case the address server, needs to possess the information concerning both the calling and the called subscriber. This, however, may be problematic if the calling and the called subscriber do not belong to the same administrative domain.
Cellular operators are used to operating autonomously within the framework of the standard interfaces, and having full control over their network elements. Operators prefer to controllably integrate separately administered subsystems by means of negotiated roaming contracts, and especially access to subscriber information is traditionally very conservatively shared. Therefore, introduction of any session establishment mechanism that requires close co-operation and continuous sharing of subscriber related information between the competing network operators is likely to face serious problems.
On the other hand, establishment of sessions in the already specified or implemented systems follows a thoroughly specified procedure, and any alterations to the elements that implement the service or to the existing network specifications are challenging, especially if the installed base is already considerable. Furthermore, changes to the functional elements of the underlying communications systems are to be considered almost impossible.